The present invention pertains to economical recovery of xenon or mixtures of xenon and krypton from air processed in a cryogenic air separation plant.
The average concentration of rare gases in atmospheric air is extremely small. For example, xenon is present in amounts of about 0.09 part per million (ppm) and krypton is present in amounts of about 1.1 ppm. In order to recover xenon and/or krypton from air it is necessary to process large volumes of air. To build a facility to produce only rare gases from air would not be economical utilizing current technology.
In practice a small stream more concentrated in xenon and/or mixtures of krypton and xenon is usually withdrawn from an oxygen plant for further treatment. Due to the fact that the volatility of krypton and xenon is lower than the volatility of oxygen, the stream is usually in a form of a liquid oxygen purge. This purge stream is then further concentrated by stripping some of the oxygen in the distillation column to produce a raw xenon or krypton and xenon stream. Because the raw stream contains other non-volatile components, there are several factors limiting the maximum degree of concentration of xenon in the raw stream. These include, among others, solubility of carbon dioxide (CO.sub.2), solubility of nitrous oxide (N2O) and the Lower Explosion Limit (LEL) of hydrocarbons present in the raw stream.
The raw stream is then subjected to a series of operations in order to purify the xenon or a krypton-xenon mixture completely by vaporizing the stream, treating the stream to remove hydrocarbons (usually by chemical reaction), removing carbon dioxide, N.sub.2 O and water (usually by adsorption) and cooling the stream to cryogenic temperature, e.g. -290.degree. F. (-179.degree. C.), for final distillation.
Due to the cost of the facility to accomplish the large number of process steps that are necessary to purify xenon or a krypton-xenon mixture, xenon recovery from small and medium oxygen plants, (e.g. up to 1000 tons per day) is not economically attractive. On the other hand, the number of small and medium oxygen plants that are either existing or are in the process of being or are recently built is relatively high, with potentially large amounts of xenon and/or krypton and xenon that are not presently being recovered. Therefore, it is the primary objective of the present invention to provide an economically attractive way to recover xenon and/or krypton and xenon from existing oxygen plants.
There is no disclosure in the prior art concerning the issues of economics of producing xenon and/or krypton-xenon mixtures as a function of the size of an air separation plant. In all of the prior art related to xenon or krypton-xenon mixture recovery, it is assumed that a recovery and purification system has to be built. The prior art describe only technical details and possible advantages of various recovery systems.
U.S. Pat. No. 3,191,393 describes a krypton/xenon separation and process consisting of an initial (raw) distillation column, a catalytic reactor, carbon dioxide separator and dryer, a batch distillation device and the necessary heat exchangers.
A similar process, with an additional distillation column for rejection of methane, is disclosed in U.S. Pat. No. 4,421,536.
U.S. Pat. No. 3,596,471 discloses a process for recovering a mixture of krypton and xenon from air with an argon stripper. Other parts of the process include hydrocarbon reactor, a CO.sub.2 separator and dryer, and a continuous distillation column for final purification.
Patentees in U.S. Pat. No. 3,609,983 disclose a krypton-xenon recovery system using a two-stage distillation process, hydrocarbon contaminant removal by adsorption and catalytic combustion with the resultant water and carbon dioxide be frozen out in heat exchangers.
U.S. Pat. No. 4,384,867 describes a more complex process for recovery of krypton and xenon, where, in addition to krypton and xenon a liquid oxygen stream is produced and an argon recycle stream is used to provide the necessary heat for rectification.
U.S. Pat. Nos. 4,401,448 and 5,067,976 disclose air separation processes for the production of krypton and xenon where the raw mixture from the first distillation column is further concentrated using a mixing column with a feed that also contains nitrogen. Therefore, the rare gases (together with hydrocarbons) are concentrated safely in a nitrogen environment, instead of oxygen.
U.S. Pat. Nos. 3,751,934; 3,768,270; 3,779,028; 4,586,528; 4,647,229; 5,122,173; 5,309,719; and 5,313,802 disclose various methods for removing hydrocarbons so they will not concentrate in to great of quantity with krypton and xenon in the bottom of the raw column. Concentration control is realized by reducing the reflux ratio in the raw distillation column by replacing the single feed to the column with various combinations of multiple feeds and/or bypasses. This permits most of the methane to be stripped and leave the raw column with the top vapor while krypton and xenon are retained in the bottom product. Also hydrocarbon adsorbers are discussed for removal of heavier hydrocarbons.
None of the prior art describes an economical process for recovery xenon and/or mixtures of krypton and xenon from small and medium size oxygen plants.